Blowout preventers (BOPs) are typically used in subsea drilling operations to protect an oil well from pressure surges in the well. Generally, BOPs include a series of rams aligned with a central bore. A drill pipe extends through the central bore and into the well below the BOP. Each set of rams is typically positioned with one ram on either side of the central bore. Some rams are designed to seal against the drill string when closed, but not to cut the drill string. Other rams include blades, and are designed to shear the drill string (and anything else in the central bore) when the rams are closed to completely seal the top of the well. These are referred to as shear rams.
A typical BOP includes a bore that runs through the BOP and connects to a wellbore. Pipe and tools are introduced to the wellbore through the bore in the BOP. Generally, blind shear rams are included in a BOP stack, and are used to shear pipe or tools inside a bore where containment of the pressure within the bore is necessary, such as in a situation where an unexpected pressure surge in the well poses a danger to personnel on a rig or other well site.
Blind shear rams typically include shear ram blocks that are mounted inside a housing, or bonnet, on the BOP. The shear ram blocks have blades that are attached to the front ends thereof, toward the bore. When the shear rams are activated, pistons push the shear ram blocks within the housing, causing the shear ram blocks and blades to close across the bore, simultaneously shearing any pipe, tools, or other objects in the bore and sealing the well. As the shear rams close, the shear ram blocks and blades are exposed to the wellbore pressure, which may be very high, such as more than 15,000 pounds per square inch (psi).
Some existing shear ram designs utilize bolts or other fasteners to attach the ram blades to the shear ram blocks. Usually such bolts are passed through the front face of the blade into the block. However, drilling holes through the face of the blade for the fastener may degrade the blade, and may introduce stress paths. In addition, the positioning of the fasteners on the blade requires staggering of the height of the bolts on the blade, leading to uneven distribution of stresses in the bolts under pressure. As a result, it is common for bolts to fracture at pressures higher than about 15,000 psi.
In today's oil and gas industry, however, drilling operations are moving into ever deeper water, which causes ever higher pressures in the wellbore. It is not uncommon, for example, for a BOP to sit on top of a well whose pressure is greater than 15,000 psi, and even up to about 20,000 psi or more, thereby exceeding the operational constraints of known BOP blind shear rams. It may be useful to provide an improved blind shear ram for a BOP suitable in withstanding high pressure and corrosive deep-water environments.